A.R.A. Zauner

Recent advances in defect-selective etching of GaN

In this communication two defect-selective etching methods for GaN are evaluated and critically compared: (i) orthodox etching in molten bases (KOH}NaOH eutectic denoted E) and in hot H 2 SO 4 /H 3 PO 4 acids (denoted HH etching) and (ii) photoelectrochemical (PEC) etching in aqueous KOH solutions. The parameters of etching are given for di!erent type of materials, i.e. bulk crystals and epitaxial layers, for both Ga- and N-polar M 0001 Nsurfaces. It is shown that molten bases are e!ective in revealing nano-pipes, inversion domains (IDs) and some dislocations, but the optimal etching parameters depend on the type of material, type and density of defects and polarity. Both orthodox etchants that result in the formation of pits on dislocations are also suitable for revealing micro-defects in heavily Mg-doped GaN single crystals but instead of pits, protruding etch features are formed. The reliability and limitations of these etching methods in revealing defects are demonstrated by means of transmission electron microscopy (TEM) calibration and by the selective formation of etch pits on dislocations introduced by indentation. Comparison with PEC etching, newly developed for GaN, is brie#y discussed. ( 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff

Morphological and structural characteristics of homoepitaxial GaN grown by metalorganic chemical vapour deposition (MOCVD)

MOCVD-grown GaN on the N-polar surface of GaN substrates has been found to exhibit gross hexagonal pyramidal features (typically 10}50 lm in size depending on layer thickness). The evolution of the pyramidal defects is dominated by the growth rate of an emergent core of inversion domain (typically 100 nm in size). The inversion domains nucleate at a thin band of oxygen containing amorphous material (2}5 nm in thickness), being remnant contamination from the mechano-chemical polishing technique used to prepare the substrates prior to growth. Apart from pyramidal hillocks, the #at-topped hillocks are also formed. The arguments are presented on the association between these features and the core dislocations, which constitute the source of the growth steps. Improvement in the substrate polishing procedures allowed the e!ective elimination of these surface hillocks. ( 1999 Elsevier Science B.V. All rights reserved.

Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001) GaN substrates. In contrast to broad PL emission in exact (0001) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.

Photoluminescence study of homoepitaxial N-polar GaN grown on differently misoriented single crystal substrates

Abstract We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal (0 0 0 1 ) GaN single crystal substrates. Off-angles of 2° and 4° towards the [1 1 2 0] direction as well as 4° in the [1 0 1 0] direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the [1 1 2 0] direction results in an N-polar epilayer of the best optical quality.

Shallow-impurity-related photoluminescence in homoepitaxial GaN

Photoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (0001) GaN substrates (i.e. N-polar) demonstrate high optical quality showing free exciton transitions in addition to narrow bound-exciton peaks (line width 1-2 meV). In contrast, N-polar films deposited on exactly oriented (0001) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (0001) epilayers reveal very high concentrations of oxygen (1.5x10 atom/cm), which are about one order of magnitude higher then in the layers grown on a 4°-off substrate. It is therefore concluded that oxygen is responsible for the high free carrier concentration and could be a dominant shallow donor in the homoepitaxial N-polar films. It is also shown that incorporation of oxygen strongly depends on the polarity of the films (Ga or N). Concentrations of other impurities, such as Si, C and H are found to be similar for homoepitaxial films of both polarities being consistent with the results of exciton-related PL.

A study on the silane doping of hetero-epitaxial MOCVD grown GaN

This paper presents a comprehensive study of SiH4 doping of GaN grown by metal-organic chemical vapour deposition (MOCVD) on sapphire substrates. The silicon incorporation appeared to be virtually independent of the growth temperature and a 1:1 relationship between the carrier concentration and the input mol fraction silane was found. Optical microscopy was used to study the morphology of the layers where photoluminescence provided a measure of the optical quality. The electrical properties were determined with Hall measurements and a comparison was made with ellipsometry measurements. The relationship between the free carrier concentration and the ellipsometric data made it possible to use ellipsometry as an alternative, quick, contactless and non-destructive technique to determine the free electron concentration in GaN layers doped with silicon.

Temperature dependent morphology transition of GaN films

The temperature dependence of the surface morphology of GaN epilayers was studied with AFM. The layers were grown by low pressure MOCVD on (0001) sapphire substrates in the temperature range of 980--1,085 C. In this range the (0001) and {l_brace}1{bar 1}01{r_brace} faces completely determine the morphology of 1.5 {micro}m thick Ga-faced GaN films. For specimens grown at 20 mbar and temperatures below 1,035 C the {l_brace}1{bar 1}01{r_brace} faces dominate the surface, which results in matt-white layers. At higher growth temperatures the morphology is completely determined by (0001) faces, which lead to smooth and transparent samples. For growth at 50 mbar, this transition takes place between 1,000 C and 1,015 C. It is shown that the morphology of the films can be described using a parameter {alpha}{sub GaN}, which is proportional to the relative growth rates of the (0001) and the {l_brace}1{bar 1}01{r_brace} faces.

Physical Properties of Silicon Doped Hetero-Epitaxial MOCVD Grown GaN: Influence of Doping Level and Stress

Silicon doped layers GaN were grown with MOCVD on sapphire substrates using silane as silicon precursor. The influence of the silicon doping concentration on the physical and optical properties is investigated. A linear relationship is found between the silane-input molfraction and the free carrier concentration in the GaN layers. The morphology of the samples is drastically changed at high silicon concentrations. Photoluminescence was used to probe bandgap variations as function of the silicon concentration. Increasing of the doping concentration led to a continuous shift of the exciton related PL to lower energies, while the intensity of the UV emission was found to increase up to a carrier concentration of n = 2.5 x 10{sup 18} cm{sup {minus}3}.